QuoteOliver Riesen
The german language file from Berti covers only Phorum core (without included modules). If you are using core modules or additional third-party-modules you can try the German Language Files Package which covers Phorum core, core modules and frequently used modules.

that has nothing to do with installing a language file.
for installing a language file you just copy the new language file to the include/lang/ folder in your phorum install.
then you can select it in the admin.